This puzzle is inspired by and has similar rules to the seriesofpuzzles by Sleafar.

Definition. A copycat chess opening is a sequence of moves starting from the conventional starting position, where every move by White (including the last one) is copied by Black identically, resulting in a symmetrical position with respect to the mid-horizontal axis. Every move must be legal, but the last move doesn't have to be checkmate, hence the name opening instead of game.

What is the shortest copycat chess opening in which a rook captures another rook?

$\begingroup$@Penguino Here descriptive is even better since you have to write each move only once (e.g. 1. P-QR4 2. P-QKt4 3. KtPxP 4. P-R6 5. P-R7 6. PxKt=R 7. RxR, I probably write it wrong, since in my country DN isn't used at all).$\endgroup$
– trolley813Apr 11 at 5:56

We can't have one of white's rooks that's already on the board take one of black's rooks across a file, since that move would not be copyable. Also, it's impossible for the rooks to capture each other along a rank, since that would require the rooks pass each other at some point, which cannot happen as they will mutually block each other each move they make.

The only other way to have a rook takes rook opening would be pawn promotion, whereby the mirrored pawns promote to a rook and then take the already-there rooks. This quickly leads us to the pawns on the a- and b-files (or g- and h-files), and thus, the solution above.

$\begingroup$@ArnaudMortier: for a horizontal rook capture a rook would have to move to the opposite side of the board. This can't be mirrored because the rooks would collide. ie assume no other pieces in the way to move a rook from a1 to a6 is possible but then the opponents rook would need to move from a8 to a3 - impossible due to the rook on a6.$\endgroup$
– ChrisApr 8 at 11:54

1

$\begingroup$@Chris True. But PilsNot has edited his answer since then to add this point of explanation.$\endgroup$
– Arnaud MortierApr 8 at 11:56

$\begingroup$It wasn't my intention to make the same puzzle with other pieces. For me the rooks are by far the most interesting because you have to find the trick, otherwise it's not possible (fastest or not).$\endgroup$
– Arnaud MortierApr 8 at 9:44

1

$\begingroup$@trolley813 well, not exactly the same thing: White rooks can't leave the lower half of the board while Black rooks can't leave the upper half. Queens, however, can move diagonally and therefore reach the opposite half of the board easily. The only reason they can't capture another Queen is that there are only two of them. If there were four like the rooks, then you wouldn't need a promotion.$\endgroup$
– Arnaud MortierApr 10 at 21:39